1. Field of the Invention
The invention relates to a process for curing aminoplast resins having improved toughness, and aminoplast resins cured in this manner.
2. Description of Related Art
Semifinished products and moldings of aminoplasts, such as compression moldings, injection moldings, profiles, fibers, foams, coatings and laminates, are known (Ullmann's Encyclopedia of Industrial Chemistry, 4th edition, Vol. A2, 115–141). A disadvantage of the semifinished products and moldings of aminoplasts is their poor toughness.
A number of processes for improving the toughness of semifinished products and moldings of aminoplasts are known.
Moldings of melamine resins having a high toughness are obtained by means of melamine resin formulations in which some of the melamine component is replaced by melamine substituted by hydroxyoxyalkyl groups (EP 0 408 947), or by means of incorporation of glycols into the melamine resin (EP 0 149 652). In the preparation of melamine resin foams, an improvement in the toughness is achieved if foaming and crosslinking are effected by exposure to microwave radiation (EP 0 037 470). The brittleness of melamine resin laminates is reduced by using melamine resins which contain dicyandiamide and polyalcohols incorporated into the melamine resin component (WO 96 20 230).
In the preparation of foams based on urea resins, foams having improved toughness are obtained if urea resins modified with polyalcohols, such as pentaerythritol, arabitol or sorbitol (DAS 1 054 232) or with polyethylene glycols (U.S. Pat. No. 2,807,595) are used.
A disadvantage of these processes is that the increase in the toughness is associated with a decrease in the strength of the semifinished products and moldings.
The toughness of semifinished products and moldings of aminoplasts is furthermore determined by the curing agents used and the degree of curing. Known curing agents for aminoplasts are p-toluenesulfonic acid, naphthalenesulfonic acid, phthalic acid, maleic acid and amine and ammonium salts of inorganic acids (Woebcken, W., Kunststoff-Handbuch [Plastics Handbook], Vol. 10, 2nd edition, Carl-Hanser-Verlag, Munich 1988). The possibilities for improving the toughness of aminoplasts with control of the course of curing by curing temperature and pH range are, however, limited.